Understanding Signature Engines

A signature engine is a component of the Cisco IPS that is designed to support many signatures in a certain category. An engine is composed of a parser and an inspector. Each engine has a set of parameters that have allowable ranges or sets of values.

Note The Cisco IPS engines support a standardized Regex.

Cisco IPS contains the following signature engines:

•AIC—Provides thorough analysis of web traffic. The AIC engine provides granular control over HTTP sessions to prevent abuse of the HTTP protocol. It allows administrative control over applications, such as instant messaging and gotomypc, that try to tunnel over specified ports. You can also use AIC to inspect FTP traffic and control the commands being issued. There are two AIC engines: AIC FTP and AIC HTTP.

•Atomic—The Atomic engines are now combined into four engines with multi-level selections. You can combine Layer 3 and Layer 4 attributes within one signature, for example IP + TCP. The Atomic engine uses the standardized Regex support.

–Atomic ARP—Inspects Layer 2 ARP protocol. The Atomic ARP engine is different because most engines are based on Layer 3 IP protocol.

–Atomic IPv6—Detects two IOS vulnerabilities that are stimulated by malformed IPv6 traffic.

•Fixed—Performs parallel regular expression matches up to a fixed depth, then stops inspection using a single regular expression table. There are three Fixed engines: ICMP, TCP, and UDP.

•Flood—Detects ICMP and UDP floods directed at hosts and networks. There are two Flood engines: Flood Host and Flood Net.

•Meta—Defines events that occur in a related manner within a sliding time interval. This engine processes events rather than packets.

•Multi String—Inspects Layer 4 transport protocols and payloads by matching several strings for one signature. This engine inspects stream-based TCP and single UDP and ICMP packets.

•Normalizer—Configures how the IP and TCP normalizer functions and provides configuration for signature events related to the IP and TCP normalizer. Allows you to enforce RFC compliance.

•Service—Deals with specific protocols. Service engine has the following protocol types:

–DNS—Inspects DNS (TCP and UDP) traffic.

–FTP—Inspects FTP traffic.

–Generic—Decodes custom service and payload, and generically analyzes network protocols.

–H225— Inspects VoIP traffic. Helps the network administrator make sure the SETUP message coming in to the VoIP network is valid and within the bounds that the policies describe. Is also helps make sure the addresses and Q.931 string fields such as url-ids, email-ids, and display information adhere to specific lengths and do not contain possible attack patterns.

Caution The SMB engine has been replaced by the SMB Advanced engine. Even though the SMB engine is still visible in IDM, IME, and the CLI, its signatures have been obsoleted; that is, the new signatures have the obsoletes parameter set with the IDs of their corresponding old signatures. Use the new SMB Advanced engine to rewrite any custom signature that were in the SMB engine.

–SNMP—Inspects SNMP traffic.

–SSH—Inspects SSH traffic.

–TNS—Inspects TNS traffic.

•State—Stateful searches of strings in protocols such as SMTP. The state engine now has a hidden configuration file that is used to define the state transitions so new state definitions can be delivered in a signature update.

•String—Searches on Regex strings based on ICMP, TCP, or UDP protocol. There are three String engines: String ICMP, String TCP, and String UDP.

•Sweep—Analyzes sweeps from a single host (ICMP and TCP), from destination ports (TCP and UDP), and multiple ports with RPC requests between two nodes. There are two Sweep engines: Sweep and Sweep Other TCP.

•Traffic Anomaly—Inspects TCP, UDP, and other traffic for worms.

•Traffic ICMP—Analyzes nonstandard protocols, such as TFN2K, LOKI, and DDOS. There are only two signatures with configurable parameters.

•Trojan—Analyzes traffic from nonstandard protocols, such as BO2K andTFN2K. There are three Trojan engines: Bo2k, Tfn2k, and UDP. There are no user-configurable parameters in these engines.

Master Engine

The Master engine provides structures and methods to the other engines and handles input from configuration and alert output. This section describes the Master engine, and contains the following topics:

1This is a static information category that you can set in the configuration and view in the alerts.Refer to the MARS documentation for more information.

Promiscuous Delta

The promiscuous delta lowers the risk rating of certain alerts in promiscuous mode. Because the sensor does not know the attributes of the target system and in promiscuous mode cannot deny packets, it is useful to lower the prioritization of promiscuous alerts (based on the lower risk rating) so the administrator can focus on investigating higher risk rating alerts. In inline mode, the sensor can deny the offending packets so that they never reach the target host, so it does not matter if the target was vulnerable. Because the attack was not allowed on the network, the IPS does not subtract from the risk rating value. Signatures that are not service, OS, or application-specific have 0 for the promiscuous delta. If the signature is specific to an OS, service, or application, it has a promiscuous delta of 5, 10, or 15 calculated from 5 points for each category.

Caution We recommend that you do NOT change the promiscuous delta setting for a signature.

Obsoletes

The Cisco signature team uses the obsoletes field to indicate obsoleted, older signatures that have been replaced by newer, better signatures, and to indicate disabled signatures in an engine when a better instance of that engine is available.

Vulnerable OS List

When you combine the vulnerable OS setting of a signature with passive OS fingerprinting, the IPS can determine if it is likely that a given attack is relevant to the target system. If the attack is found to be relevant, the risk rating value of the resulting alert receives a boost. If the relevancy is unknown, usually because there is no entry in the passive OS fingerprinting list, then no change is made to the risk rating. If there is a passive OS fingerprinting entry and it does not match the vulnerable OS setting of a signature, the risk rating value is decreased. The default value by which to increase or decrease the risk rating is +/- 10 points.

Alert Frequency

The purpose of the alert frequency parameter is to reduce the volume of the alerts written to the Event Store to counter IDS DoS tools, such as stick. There are four modes: Fire All, Fire Once, Summarize, and Global Summarize. The summary mode is changed dynamically to adapt to the current alert volume. For example, you can configure the signature to Fire All, but after a certain threshold is reached, it starts summarizing.

Summarizes an alert so that it only fires once regardless of how many attackers or victims.

—

Summarize

Summarizes alerts.

—

Summary Threshold

Threshold number of alerts to send signature into summary mode.

0 to 65535

Global Summary Threshold

Threshold number of events to take alerts into global summary.

1 to 65535

Summary Interval

Time in seconds used in each summary alert.

1 to 1000

Summary Key

The storage type on which to summarize this signature:

•Attacker address

•Attacker and victim addresses

•Attacker address and victim port

•Victim address

•Attacker and victim addresses and ports

Axxx

AxBx

Axxb

xxBx

AaBb

Event Actions

Note Most of the following event actions belong to each signature engine unless they are not appropriate for that particular engine.

The following event action parameters belong to each signature engine (if it makes sense for that signature engine):

•Alert and Log Actions

–Product Alert—Writes an alert to Event Store.

Note The Product Alert action is not automatic when you enable alerts for a signature. To have an alert created in the Event Store, you must select Product Alert. If you add a second action, you must include Product Alert if you want an alert sent to the Event Store. Also, every time you configure the event actions, a new list is created and it replaces the old list. Make sure you include all the event actions you need for each signature.

Note A Produce Alert event action is added for an event when global correlation has increased the risk rating of an event, and has added either the Deny Packet Inline or Deny Attacker Inline event action.

–Produce Verbose Alert—Includes an encoded dump (possibly truncated) of the offending packet in the alert.

Note You cannot delete the event action override for Deny Packet Inline because it is protected. If you do not want to use that override, disable it.

–Deny Connection Inline—(inline mode only) Does not transmit this packet and future packets on the TCP Flow.

–Deny Attacker Victim Pair Inline—(inline mode only) Does not transmit this packet and future packets on the attacker/victim address pair for a specified period of time.

–Deny Attacker Service Pair Inline—(inline mode only) Does not transmit this packet and future packets on the attacker address victim port pair for a specified period of time.

–Deny Attacker Inline—(inline mode only) Does not transmit this packet and future packets from the attacker address for a specified period of time.

Note This is the most severe of the deny actions. It denies the current and future packets from a single attacker address. Each deny address times out for X seconds from the first event that caused the deny to start, where X is the amount of seconds that you configured. You can clear all denied attacker entries by choosing Monitoring > Properties > Denied Attackers > Clear List, which permits the addresses back on the network.

–Modify Packet Inline—(inline mode only) Modifies packet data to remove ambiguity about what the end point might do with the packet.

Note Modify Packet Inline is part of the Normalizer Engine. It scrubs the packet and corrects irregular issues such as bad checksum, out of range values, and other RFC violations.

•Other Actions

Note IPv6 does not support the following event actions: Request Block Host, Request Block Connection, or Request Rate Limit.

For signatures that have Deny Packet Inline configured as an action or for an event action override that adds Deny Packet Inline as an action, the following actions may be taken:

•droppedPacket

•deniedFlow

•tcpOneWayResetSent

The deny packet inline action is represented as a dropped packet action in the alert. When a deny packet inline occurs for a TCP connection, it is automatically upgraded to a deny connection inline action and seen as a denied flow in the alert. If the IPS denies just one packet, the TCP continues to try to send that same packet again and again, so the IPS denies the entire connection to ensure it never succeeds with the resends.

When a deny connection inline occurs, the IPS also automatically sends a TCP one-way reset, which shows up as a TCP one-way reset sent in the alert. When the IPS denies the connection, it leaves an open connection on both the client (generally the attacker) and the server (generally the victim). Too many open connections can result in resource problems on the victim. So the IPS sends a TCP reset to the victim to close the connection on the victim side (usually the server), which conserves the resources of the victim. It also prevents a failover that would otherwise allow the connection to fail over to a different network path and reach the victim. The IPS leaves the attacker side open and denies all traffic from it.

Regular Expression Syntax

Regular expressions (Regex) are a powerful and flexible notational language that allow you to describe text. In the context of pattern matching, regular expressions allow a succinct description of any arbitrary pattern.

Understanding the AIC Engine

AIC provides thorough analysis of web traffic. It provides granular control over HTTP sessions to prevent abuse of the HTTP protocol. It allows administrative control over applications, such as instant messaging and gotomypc, that try to tunnel over specified ports. Inspection and policy checks for P2P and instant messaging are possible if these applications are running over HTTP.

AIC also provides a way to inspect FTP traffic and control the commands being issued.

You can enable or disable the predefined signatures or you can create policies through custom signatures.

Note The AIC engine runs when HTTP traffic is received on AIC web ports. If traffic is web traffic, but not received on the AIC web ports, the Service HTTP engine is executed. AIC inspection can be on any port if it is configured as an AIC web port and the traffic to be inspected is HTTP traffic.

AIC Engine and Sensor Performance

Application policy enforcement is a unique sensor feature. Rather than being based on traditional IPS technologies that inspect for exploits, vulnerabilities, and anomalies, AIC policy enforcement is designed to enforce HTTP and FTP service policies. The inspection work required for this policy enforcement is extreme compared with traditional IPS inspection work. A large performance penalty is associated with using this feature. When AIC is enabled, the overall bandwidth capacity of the sensor is reduced.

AIC policy enforcement is disabled in the IPS default configuration. If you want to activate AIC policy enforcement, we highly recommend that you carefully choose the exact policies of interest and disable those you do not need. Also, if your sensor is near its maximum inspection load capacity, we recommend that you not use this feature since it can oversubscribe the sensor. We recommend that you use the adaptive security appliance firewall to handle this type of policy enforcement.

AIC Engine Parameters

The AIC engine defines signatures for deep inspection of web traffic. It also defines signatures that authorize and enforce FTP commands.

There are two AIC engines: AIC HTTP and AIC FTP.

The AIC engine has the following features:

•Web traffic:

–RFC compliance enforcement

–HTTP request method authorization and enforcement

–Response message validation

–MIME type enforcement

–Transfer encoding type validation

–Content control based on message content and type of data being transferred

–URI length enforcement

–Message size enforcement according to policy configured and the header

–Tunneling, P2P and instant messaging enforcement.

This enforcement is done using regular expressions. There are predefined signature but you can expand the list.

•FTP traffic:

–FTP command authorization and enforcement

Table B-5 lists the parameters that are specific to the AIC HTTP engine.

Table B-5 AIC HTTP Engine Parameters

Parameter

Description

Signature Type

The type of AIC signature.

Content Types

AIC signature that deals with MIME types:

•Define Content Type—Associates actions such as denying a specific MIME type (image/gif), defining a message-size violation, and determining that the MIME-type mentioned in the header and body do not match.

Atomic ARP Engine

Table B-7 lists the parameters that are specific to the Atomic ARP engine.

Table B-7 Atomic ARP Engine Parameters

Parameter

Description

Value

Specify ARP Operation

(Optional) Enables ARP operation:

•ARP Operation—Type of ARP operation to inspect.

0 to 65535

Specify Mac Flip Times

(Optional) Enables MAC address flip times:

•Mac Flip Times—Specifies how many times to flip the MAC address in the alert.

0 to 65535

Specify Request Inbalance

(Optional) Enables request inbalance:

•Request Inbalance—Fires an alert when there are this many more requests than replies on the IP address.

0 to 65535

Specify Type of ARP Sig

(Optional) Enables type of ARP signatures:

•Type of ARP Sig—Specifies the type of ARP signatures you want to fire on:

–Destination Broadcast—Fires an alert for this signature when it sees an ARP destination address of 255.255.255.255.

–Same Source and Destination—Fires an alert for this signature when it sees an ARP destination address with the same source and destination MAC address

–Source Broadcast (default)—Fires an alert for this signature when it sees an ARP source address of 255.255.255.255.

–Source Multicast—Fires an alert for this signature when it sees an ARP source MAC address of 01:00:5e:(00-7f).

Dst Broadcast

Same Src and Dst

Src Broadcast

Src Multicast

Storage Key

Type of address key used to store persistent data:

•Attacker address

•Attacker and victim addresses

•Victim address

•Global

Axxx

AxBx

xxBx

xxxx

Atomic IP Advanced Engine

The Atomic IP Advanced engine parses and interprets the IPv6 header and its extensions, the IPv4 header and its options, ICMP, ICMPv6, TCP, and UDP, and seeks out anomalies that indicate unusual activity.

Atomic IP Advanced engine signatures do the following:

•Inspect for anomalies in IP addresses, for example, spoofed addresses

•Inspect for bad information in the length fields of the packet

•Fire informational alerts about the packet

•Fire higher severity alerts for the limited set of known vulnerabilities

•Duplicate any IPv6-specific signatures in Engine Atomic IP that can also apply to IPv6

•Provide default signatures for identifying tunneled traffic based on IP address, port, protocol, and limited information from the packet data.

Only the outermost IP tunnel is identified. When an IPv6 tunnel or IPv6 traffic inside of an IPv4 tunnel is detected, a signature fires an alert. All of the other IPv6 traffic in embedded tunnels is not inspected. The following tunneling methods are supported, but not individually detected. For example, ISATAP, 6to4, and manual IPv6 RFC 4213 tunnels all appear as IPv6 in IPv4, which is detected by signature 1007:

•Cannot detect the Layer 4 field of the packets if the packets are fragmented so that the Layer 4 identifier does not appear in the first packet.

•Cannot detect Layer 4 attacks in flows with packets that are fragmented by IPv6 because there is no fragment reassembly.

•Cannot detect attacks with tunneled flows.

•Limited check are provided for the fragmentation header.

•Although you can configure IPv6 setting on all platforms, the IPS modules (AIM IPS, AIP SSM, IDSM2, and NME IPS) do not support the IPv6 features because the router, adaptive security appliance, or switch in which they are installed do not send them IPv6 data.

•If there are illegal duplicate headers, a signature fires, but the individual headers cannot be separately inspected.

•IPv6 does not support the following event actions: Request Block Host, Request Block Connection, or Request Rate Limit.

•Anomaly detection does not support IPv6 traffic; only IPv4 traffic is directed to the anomaly detection processor.

•Rate limiting and blocking are not supported for IPv6 traffic. If a signature is configured with a block or rate limit event action and is triggered by IPv6 traffic, an alert is generated but the action is not carried out.

Atomic IP Advanced Engine Parameters

Note The second number in the ranges must be greater than or equal to the first number.

Table B-8 lists the parameters that are specific to the Atomic IP Advanced engine.

Table B-8 Atomic IP Advanced Engine Parameters

Parameter

Description

Value

Global

Fragment Status

Specifies whether or not fragments are wanted.

Any | No Fragments | Want Fragments

Specify Encapsulation

(Optional) Specifies any encapsulation before the start of L3 for the packet:1

•Encapsulation—Type of encapsulation for which to inspect.

None | MPLS | GRE | Ipv4 in IPv6 | IP IP | Any

Specify IP Version

(Optional) Specifies IP protocol version:

•IP Version—IPv4 or IPv6.

IPv4| IPv6

Swap Attacker Victim

Swaps the attacker and victim addresses and ports (source and destination) in the alert message and for any actions taken. The default is No.

Yes | No

Regex

Specify Regex Inspection

(Optional) Enables Regex inspection.

Yes | No

Regex Scope

Specifies the start and end points for the search.

•ipv6-doh-only

•ipv6-doh-plus

•ipv6-hoh-only

•ipv6-hoh-plus

•ipv6-rh-only

•ipv6-rh-plus

•layer3-only

•layer3-plus

•layer4

Regex String

Specifies the regular expression to search for in a single TCP packet.

string

Specify Exact Match Offset

Enables exact match offset:

•Exact Match Offset—The exact stream offset the Regex String must report for a match to be valid.

0 to 65535

Specify Minimum Match Length

Enables minimum match length:

•Minimum Match Length—Specifies the minimum number of bytes the Regex String must match.

0 to 65535

Specify Minimum Match Offset

Enables minimum match offset:

•Minimum Match Offset—Specifies the minimum stream offset the Regex String must report for a match to be valid.

0 to 65535

Specify Maximum Match Offset

Enables maximum match offset:

•Maximum Match Offset—Specifies the maximum stream offset the Regex String must report for a match to be valid.

0 to 65535

IPv6

Specify Authentication Header

(Optional) Enables inspection of the authentication header:

•AH Present—Specifies that the authentication header is present:

–AH Length—Specifies the length of the authentication header.

–AH Next Header—Specifies the value of the authentication header.

Have AH | No AH

0 to 1028

0 to 255

Specify Destination Options Header

(Optional) Enables inspection of the destination options header:

•DOH Present—Specifies that the destination options header is present:

–DOH Count—Specifies the number of destination options headers for which to inspect.

–DOH Length—Specifies the length of destination options headers for which to inspect.

–DOH Next Header—Specifies the number of next destination options headers for which to inspect.

–DOH Option Type—Specifies the type of destination options headers for which to inspect.

–DOH Option Length—Specifies the length of destination options headers for which to inspect.

Have DOH | No DOH

0 to 2

8 to 2048

0 to 255

0 to 255

0 to 255

Specify ESP Header

(Optional) Enables inspection of the ESP header:

•ESP Present—Specifies that the ESP header is present.

Have ESP | No ESP

Specify First Next Header

(Optional) Enables inspection of the first next header:

•First Next Header—Specifies the value of the first next header for which to inspect.

0 to 255

Specify Flow Label

(Optional) Enables inspection of the flow label:

•Flow Label—Specifies the value of the flow label for which to inspect.

0 to 1048575

Specify Headers Out of Order

(Optional) Enables inspection of out-of-order headers:

•Headers Out of Order—Specifies the header order for which to inspect.

Yes | No

Specify Headers Repeated

(Optional) Enables inspection of repeated headers:

•Headers Repeated—Specifies the header repetition for which to inspect.

Yes | No

Specify Hop Limit

(Optional) Enables hop limit:

•Hop Limit—Specifies the value of the hop limit for which to inspect.

0 to 255

Specify Hop Options Header

(Optional) Enables inspection of the hop-by-hop options header:

•HOH Present—Specifies that the hop-by-hop options header is present.

Have HOH | No HOH

Specify IPv6 Address Options

(Optional) Enables the IPv6 address options:

•IPv6 Address Options—Specifies the IPv6 address options:

–Address With Localhost—IP address with ::1.

–Documentation Address—IP address with 2001:db8::/32 prefix.

–IPv6 Address—IP address.

–Link Local Address—Inspects for an IPv6 link local address.

–Multicast Destination—Inspects for a destination multicast address.

–Multicast Source—Inspects for a source multicast address.

–Not Link Local Address—Inspects for an address that is not link-local.

–Not Valid Address—Inspects for an address that is not reserved for link-local, global, or multicast.

•IP Header Length—Specifies the length of the IP header for which to inspect.

0 to 16

Specify IP Identifier

(Optional) Enables inspection of the IP identifier:

•IP Identifier—Specifies the IP ID for which to inspect.

0 to 255

Specify IP Option Inspection

(Optional) Enables inspection of the IP options:

•IP Option Inspection—Specifies the value of the IP option:

–IP Option—IP OPTION code to match.

–IP Option Abnormal Options—The list of options is malformed.

0 to 65535

Yes | No

Specify IP Payload Length

(Optional) Enables inspection of the IP payload length:

•IP Payload Length—Specifies the length of IP payload for which to inspect.

0 to 65535

Specify IP Type of Service

(Optional) Specifies the IP type of service:

•IP Type of Service—Specifies the IP type of service for which to inspect.

0 to 255

Specify IP Total Length

(Optional) Enables inspection of the IP total length:

•IP Total Length—Specifies the total length of iP packet for which to inspect.

0 to 65535

Specify IP Time-to-Live

(Optional) Enables inspection of the IP time-to-live:

•IP Time-to-Live—Specifies IP TTL inspection.

0 to 255

Specify IP Version

(Optional) Enables inspection of the IP version:

•IP Version—Specifies which IP version for which to inspect.

0 to 16

L4 Protocol

Specify L4 Protocol

(Optional) Enables inspection of L4 protocol:

•L4 Protocol—Specifies which L4 protocol to inspect.

ICMP Protocol

ICMPv6 Protocol

TCP Protocol

UDP Protocol

Other IP Protocols

L4 Protocol Other

Other IP Protocol ID

(Optional) Enables inspection of other L4 protocols:

•Other IP Protocol ID—Specifies which single IP protocol ID or single range of IP protocol IDs for which to send alerts.

0 to 255

L4 Protocol ICMP

Specify ICMP Code

(Optional) Enables inspection of L4 ICMP code:

•ICMP Code—Specifies ICMP header CODE value.

0 to 65535

Specify ICMP ID

(Optional) Enables inspection of L4 ICMP ID:

•ICMP ID—Specifies ICMP header IDENTIFIER value.

0 to 65535

Specify ICMP Sequence

(Optional) Enables inspection of L4 ICMP sequence:

•ICMP Sequence—Specifies the ICMP sequence for which to look.

0 to 65535

Specify ICMP Type

(Optional) Enables inspection of the ICMP header type:

•ICMP Type—Specifies the ICMP header TYPE value.

0 to 65535

L4 Protocol ICMPv6

Specify ICMPv6 Code

(Optional) Enables inspection of L4 ICMPv6 code:

•ICMPv6 Code—Specifies ICMPv6 header CODE value.

0 to 255

Specify ICMPv6 ID

(Optional) Enables inspection of the L4 ICMPv6 identifier:

•ICMPv6 ID—Specifies ICMPv6 header IDENTIFIER value.

0 to 65535

Specify ICMPv6 Length

(Optional) Enables inspection of L4 ICMPv6 length:

•ICMPv6 Length—ICMPv6 header LENGTH value.

0 to 65535

Specify ICMPv6 MTU Field

(Optional) Enables inspection of the L4 ICMPv6 MTU field:

•ICMPv6 MTU Field—ICMPv6 header MTU field value.

4,294,967,295

Specify ICMPv6 Option Type

(Optional) Enables inspection of L4 ICMPv6 type:

•ICMPv6 Option Type—Specifies which ICMPv6 option type to inspect.

0 to 255

Specify ICMPv6 Option Length

(Optional) Enables inspection of L4 ICMPv6 option type:

•ICMPv6 Option Length—Specifies which ICMPv6 option type to inspect.

0 to 255

Specify ICMPv6 Sequence

(Optional) Enables inspection of L4 ICMPv6 sequence:

•ICMPv6 Sequence—ICMPv6 header SEQUENCE value.

0 to 65535

Specify ICMPv6 Type

(Optional) Enables inspection of L4 ICMPv6 type:

•ICMPv6 Type—ICMPv6 header TYPE value.

0 to 255

L4 Protocol TCP and UDP

Specify Destination Port

(Optional) Enables the destination port for use:

•Destination Port—Destination port of interest for this signature.

0 to 65535

Specify Source Port

(Optional) Enables source port for use:

•Source Port—Source port of interest for this signature.

0 to 65535

Specify TCP Mask

(Optional) Enables the TCP mask for use:

•TCP Mask—Mask used in TCP flags comparison:

–URG bit

– ACK bit

– PSH bit

– RST bit

– SYN bit

– FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Specify TCP Flags

(Optional) Enables TCP flags for use:

•TCP Flags—TCP flags to match when masked by mask:

–URG bit

–ACK bit

–PSH bit

–RST bit

–SYN bit

–FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Specify TCP Reserved

(Optional) Enables TCP reserved for use:

•TCP Reserved—TCP reserved.

0 to 63

Specify TCP Header Length

(Optional) Enables inspection of L4 TCP header length:

•TCP Header Length—Specifies length of TCP header used in inspection.

0 to 60

Specify TCP Payload Length

(Optional) Enables inspection of L4 TCP payload length:

•TCP Payload Length—Specifies length of TCP payload.

0 to 65535

Specify TCP URG Pointer

(Optional) Enables inspection of the L4 TCP URG pointer:

•TCP URG Pointer—Specifies TCP URG flag inspection.

0 to 65535

Specify TCP Window Size

(Optional) Enables inspection of L4 TCP window size:

•TCP Window Size—Specifies the window size of the TCP packet.

0 to 65535

Specify UDP Valid Length

(Optional) Enables inspection of the L4 UDP valid length:

•UDP Valid Length—Specifies UDP packet lengths that are considered valid and should not be inspected.

0 to 65535

Specify UDP Length Mismatch

(Optional) Enables inspection of L4 UDP length mismatch:

•UDP Length Mismatch—Fires an alert when IP Data length is less than the UDP Header length.

Yes | No

1When a packet is GRE, IPIP, IPv4inIPv6, or MPL the sensor skips the L3 encapsulation header and the encapsulation header, and all inspection is done starting from the second L3. The encapsulation enumerator allows the engine to look backward to see if there is an encapsulation header before the L3 in question.

2Use the following syntax: x.x.x.x-z.z.z.z, for example, 10.10.10.1-10.10.10.254.

•IP Header Length—Specifies the length of the IP header for which to inspect.

0 to 16

Specify IP Identifier

(Optional) Enables inspection of the IP identifier:

•IP Identifier—Specifies the IP ID for which to inspect.

0 to 255

Specify IP Option Inspection

(Optional) Enables inspection of the IP options:

•IP Option Inspection—Specifies the value of the IP option:

–IP Option—IP OPTION code to match.

–IP Option Abnormal Options—The list of options is malformed.

0 to 65535

Yes | No

Specify IP Payload Length

(Optional) Enables inspection of the IP payload length:

•IP Payload Length—Specifies the length of IP payload for which to inspect.

0 to 65535

Specify IP Type of Service

(Optional) Specifies the IP type of service:

•IP Type of Service—Specifies the IP type of service for which to inspect.

0 t6o 255

Specify IP Total Length

(Optional) Enables inspection of the IP total length:

•IP Total Length—Specifies the total length of iP packet for which to inspect.

0 to 65535

Specify IP Time-to-Live

(Optional) Enables inspection of the IP time-to-live:

•IP Time-to-Live—Specifies IP TTL inspection.

0 to 255

Specify IP Version

(Optional) Enables inspection of the IP version:

•IP Version—Specifies which IP version for which to inspect.

0 to 16

Specify L4 Protocol

(Optional) Enables inspection of L4 protocol:

•L4 Protocol—Specifies which L4 protocol to inspect.

ICMP Protocol

TCP Protocol

UDP Protocol

Other IP Protocols

Specify ICMP Code

(Optional) Enables inspection of L4 ICMP code:

•ICMP Code—Specifies ICMP header CODE value.

0 to 65535

Specify ICMP ID

(Optional) Enables inspection of L4 ICMP ID:

•ICMP ID—Specifies ICMP header IDENTIFIER value.

0 to 65535

Specify ICMP Sequence

(Optional) Enables inspection of L4 ICMP sequence:

•ICMP Sequence—Specifies the ICMP sequence for which to look.

0 to 65535

Specify ICMP Type

(Optional) Enables inspection of the ICMP header type:

•ICMP Type—Specifies the ICMP header TYPE value.

0 to 65535

Specify ICMP Total Length

(Optional) Enables inspection of L4 ICMP total header length:

•ICMP Total Length—Specifies the ICMP total length value for which to inspect.

0 to 65535

Other IP Protocol ID

(Optional) Enables inspection of other L4 protocols:

•Other IP Protocol ID—Specifies which single IP protocol ID or single range of IP protocol IDs for which to send alerts.

0 to 255

Specify Destination Port

(Optional) Enables the destination port for use:

•Destination Port—Destination port of interest for this signature.

0 to 65535

Specify Source Port

(Optional) Enables source port for use:

•Source Port—Source port of interest for this signature.

0 to 65535

Specify TCP Mask

(Optional) Enables the TCP mask for use:

•TCP Mask—Mask used in TCP flags comparison:

–URG bit

– ACK bit

– PSH bit

– RST bit

– SYN bit

– FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Specify TCP Flags

(Optional) Enables TCP flags for use:

•TCP Flags—TCP flags to match when masked by mask:

–URG bit

–ACK bit

–PSH bit

–RST bit

–SYN bit

–FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Specify TCP Reserved

(Optional) Enables TCP reserved for use:

•TCP Reserved—TCP reserved.

0 to 63

Specify TCP Header Length

(Optional) Enables inspection of L4 TCP header length:

•TCP Header Length—Specifies length of TCP header used in inspection.

0 to 60

Specify TCP Payload Length

(Optional) Enables inspection of L4 TCP payload length:

•TCP Payload Length—Specifies length of TCP payload.

0 to 65535

Specify TCP URG Pointer

(Optional) Enables inspection of the L4 TCP URG pointer:

•TCP URG Pointer—Specifies TCP URG flag inspection.

0 to 65535

Specify TCP Window Size

(Optional) Enables inspection of L4 TCP window size:

•TCP Window Size—Specifies the window size of the TCP packet.

0 to 65535

Specify UDP Length

(Optional) Enables inspection of L4 UDP length:

•UDP Length—Fires an alert when IP Data length is less than the UDP Header length.

0 to 65535

Specify UDP Valid Length

(Optional) Enables inspection of the L4 UDP valid length:

•UDP Valid Length—Specifies UDP packet lengths that are considered valid and should not be inspected.

0 to 65535

Specify UDP Length Mismatch

(Optional) Enables inspection of L4 UDP length mismatch:

•UDP Length Mismatch—Fires an alert when IP Data length is less than the UDP Header length.

Yes | No

1Use the following syntax: x.x.x.x-z.z.z.z, for example, 10.10.10.1-10.10.10.254.

Atomic IPv6 Engine

The Atomic IPv6 engine detects two IOS vulnerabilities that are stimulated by malformed IPv6 traffic. These vulnerabilities can lead to router crashes and other security issues. One IOS vulnerability deals with multiple first fragments, which cause a buffer overflow. The other one deals with malformed ICMPv6 Neighborhood Discovery options, which also cause a buffer overflow.

Note IPv6 increases the IP address size from 32 bits to 128 bits, which supports more levels of addressing hierarchy, a much greater number of addressable nodes, and autoconfiguration of addresses.

There are eight Atomic IPv6 signatures. The Atomic IPv6 inspects Neighborhood Discovery protocol of the following types:

•Type 133—Router Solicitation

•Type 134—Router Advertisement

•Type 135—Neighbor Solicitation

•Type 136—Neighbor Advertisement

•Type 137—Redirect

Note Hosts and routers use Neighborhood Discovery to determine the link-layer addresses for neighbors known to reside on attached links and to quickly purge cached values that become invalid. Hosts also use Neighborhood Discovery to find neighboring routers that will forward packets on their behalf.

Each Neighborhood Discovery type can have one or more Neighborhood Discovery options. The Atomic IPv6 engine inspects the length of each option for compliance with the legal values stated in RFC 2461. Violations of the length of an option results in an alert corresponding to the option type where the malformed length was encountered (signatures 1601 to 1605).

Note The Atomic IPv6 signatures do not have any specific parameters to configure.

Fixed Engine

Understanding the Fixed Engine

The Fixed engine combines multiple regular expression patterns in to a single pattern matching table that allows a single search through the data. It supports ICMP, TCP, and UDP protocols. After a minimum inspection depth is reached (1 to 100 bytes), inspection stops. There are three Fixed engines: Fixed ICMP, Fixed TCP, and Fixed UDP.

Note Fixed TCP and Fixed UDP use the Service Ports parameter as exclusion ports. Fixed ICMP uses the Service Ports parameter as excluded ICMP types.

Flood Engine

The Flood engine defines signatures that watch for any host or network sending multiple packets to a single host or network. For example, you can create a signature that fires when 150 or more packets per second (of the specific type) are found going to the victim host. There are two types of Flood engines: Flood Host and Flood Net.

Meta Engine

Caution A large number of Meta signatures could adversely affect overall sensor performance.

Note The Meta engine enhancement is available in IPS 7.0(2)E4 and later.

The Meta engine defines events that occur in a related manner within a sliding time interval. This engine processes events rather than packets. As signature events are generated, the Meta engine inspects them to determine if they match any or several Meta definitions. The Meta engine generates a signature event after all requirements for the event are met.

All signature events are handed off to the Meta engine by the Signature Event Action Processor. The Signature Event Action Processor hands off the event after processing the minimum hits option. Summarization and event action are processed after the Meta engine has processed the component events.

Component Signatures and the Meta Engine

Component signatures are not independent signatures, they are pieces of a Meta signature. The Signature Type option is marked as Component. Since these signatures are not independent signatures, the risk rating when triggered is automatically set to 0. The risk rating is applicable to the Meta signature rather than the component signatures. This prevents the component signatures from causing denial of packets by either event action overrides or global correlation. Event action overrides and global correlation are applied against the Meta signature rather than the component signature.

Note Some component signatures in the Meta signatures are valuable as both independent signatures and component signatures. These signatures are not marked as Signature Type Component and instead are marked with the Signature Type set to either Vulnerability, Exploit, Anomaly, or Other. The risk rating for these signatures is calculated and is not set to 0.

Meta Signature Engine Enhancement

The purpose of the Meta engine is to detect a specified payload from an attacker and a corresponding payload from the victim. It is also used to inspect streams at different offsets. The Meta engine supports the AND and OR logical operators. ANDNOT capability has been added to the Meta engine. This clause is a negative clause used to complement the existing positive clause-based signatures. The previous signature format had the following form:

IF (A and B and C) then Alarm; alternatively, IF (A or B or C) then Alarm is also
supported; where A, B, and C are meta component signatures.

The addition of the negative clause allows for the following logic:

IF (A and/or B) AND NOT (C and/or D) then Alarm.

The (C and/or D) is the negative clause and is satisfied if (C and D) [alternatively (C or D)] do not occur before the Meta Reset Interval time expires.

A component of the positive clause must occur before the negative clause(s) to establish the Meta tracking state. The Meta engine cannot track the lack of past behavior. The state of the negative clause is evaluated when the Meta Reset Interval time expires.

Multi String Engine

Caution The Multi String engine can have a significant impact on memory usage.

The Multi String engine lets you define signatures that inspect Layer 4 transport protocol (ICMP, TCP, and UDP) payloads using multiple string matches for one signature. You can specify a series of regular expression patterns that must be matched to fire the signature. For example, you can define a signature that looks for regex 1 followed by regex 2 on a UDP service. For UDP and TCP you can specify port numbers and direction. You can specify a single source port, a single destination port, or both ports. The string matching takes place in both directions.

Use the Multi String engine when you need to specify more than one regex pattern. Otherwise, you can use the String ICMP, String TCP, or String UDP engine to specify a single Regex pattern for one of those protocols.

Exact number of bytes that must be between this regex string and the one before, or from the beginning of the stream/packet if it is the first entry in the list.

0 to 4294967296

Minimum Spacing

Minimum number of bytes that must be between this regex string and the one before, or from the beginning of the stream/packet if it is the first entry in the list.

0 to 4294967296

Swap Attacker Victim

Swaps the attacker and victim addresses and ports (source and destination) in the alert message and for any actions taken. The default is No.

Yes | No

1Port matching is performed bidirectionally for both the client-to-server and server-to-client traffic flow directions. For example, if the source-ports value is 80, in a client-to-server traffic flow direction, inspection occurs if the client port is 80. In a server-to-client traffic flow direction, inspection occurs if the server port is port 80.

2A valid value is a comma- separated list of integer ranges a-b[,c-d] within 0 to 65535. The second number in the range must be greater than or equal to the first number.

Normalizer Engine

Note You cannot add custom signatures to the Normalizer engine. You can tune the existing ones.

The Normalizer engine deals with IP fragment reassembly and TCP stream reassembly. With the Normalizer engine you can set limits on system resource usage, for example, the maximum number of fragments the sensor tries to track at the same time. Sensors in promiscuous mode report alerts on violations. Sensors in inline mode perform the action specified in the event action parameter, such as Produce Alert, Deny Packet Inline, and Modify Packet Inline.

Caution For signature 3050 Half Open SYN Attack, if you choose Modify Packet Inline as the action, you can see as much as 20 to 30% performance degradation while the protection is active. The protection is only active during an actual SYN flood.

IP Fragmentation Normalization

Intentional or unintentional fragmentation of IP datagrams can hide exploits making them difficult or impossible to detect. Fragmentation can also be used to circumvent access control policies like those found on firewalls and routers. And different operating systems use different methods to queue and dispatch fragmented datagrams. If the sensor has to check for all possible ways that the end host can reassemble the datagrams, the sensor becomes vulnerable to DoS attacks. Reassembling all fragmented datagrams inline and only forwarding completed datagrams, refragmenting the datagram if necessary, prevents this. The IP Fragmentation Normalization unit performs this function.

TCP Normalization

Through intentional or natural TCP session segmentation, some classes of attacks can be hidden. To make sure policy enforcement can occur with no false positives and false negatives, the state of the two TCP endpoints must be tracked and only the data that is actually processed by the real host endpoints should be passed on. Overlaps in a TCP stream can occur, but are extremely rare except for TCP segment retransmits. Overwrites in the TCP session should not occur. If overwrites do occur, someone is intentionally trying to elude the security policy or the TCP stack implementation is broken. Maintaining full information about the state of both endpoints is not possible unless the sensor acts as a TCP proxy. Instead of the sensor acting as a TCP proxy, the segments are ordered properly and the normalizer looks for any abnormal packets associated with evasion and attacks.

IPv6 Fragments

The Normalizer engine can reassemble IPv6 fragments and forward the reassembled buffer for inspection and actions by other engines and processors. The following differences exist between IPv4 and IPv6:

•Modify Packet Inline for Normalizer engine signatures has no effect on IPv6 datagrams.

You receive the following warning if you disable a default-enabled TCP Normalizer signature or remove a default-enabled modify packet inline, deny packet inline, or deny connection inline action:

Use caution when disabling, retiring, or changing the event action settings of a <Sig ID>
TCP Normalizer signature for a sensor operating in IPS mode. The TCP Normalizer signature
default values are essential for proper operation of the sensor.

If the sensor is seeing duplicate packets, consider assigning the traffic to multiple
virtual sensors. If you are having problems with asymmetric or out-of-order TCP packets,
consider changing the normalizer mode from strict evasion protection to asymmetric mode
protection. Contact Cisco TAC if you require further assistance.

AIP SSM and the Normalizer Engine

The majority of the features in the Normalizer engine are not used on the AIP SSM, because the ASA itself handles the normalization. Packets on the ASA IPS modules go through a special path in the Normalizer that only reassembles fragments and puts packets in the right order for the TCP stream. The Normalizer does not do any of the normalization that is done on an inline IPS appliance, because that causes problems in the way the ASA handles the packets.

The following Normalizer engine signatures are not supported:

•1300.0

•1304.0

•1305.0

•1307.0

•1308.0

•1309.0

•1311.0

•1315.0

•1316.0

•1317.0

•1330.0

•1330.1

•1330.2

•1330.9

•1330.10

•1330.12

•1330.14

•1330.15

•1330.16

•1330.17

•1330.18

Normalizer Engine Parameters

Table B-18 lists the parameters that are specific to the Normalizer engine.

Understanding Service Engines

The Service engines analyze Layer 5+ traffic between two hosts. These are one-to-one signatures that track persistent data. The engines analyze the Layer 5+ payload in a manner similar to the live service. The Service engines have common characteristics but each engine has specific knowledge of the service that it is inspecting. The Service engines supplement the capabilities of the generic string engine specializing in algorithms where using the string engine is inadequate or undesirable.

Service DNS Engine

The Service DNS engine specializes in advanced DNS decode, which includes anti-evasive techniques, such as following multiple jumps. It has many parameters such as lengths, opcodes, strings, and so forth. The Service DNS engine is a biprotocol inspector operating on both TCP and UDP port 53. It uses the stream for TCP and the quad for UDP.

Service FTP Engine

The Service FTP engine specializes in FTP port command decode, trapping invalid port commands and the PASV port spoof. It fills in the gaps when the String engine is not appropriate for detection. The parameters are Boolean and map to the various error trap conditions in the port command decode. The Service FTP engine runs on TCP ports 20 and 21. Port 20 is for data and the Service FTP engine does not do any inspection on this. It inspects the control transactions on port 21.

Table B-20 lists the parameters that are specific to the Service FTP engine.

Swaps the attacker and victim addresses and ports (source and destination) in the alert message and for any actions taken. The default is No.

Yes | No

1The second number in the range must be greater than or equal to the first number.

Service Generic Engine

Note You cannot use the Service Generic engine to create custom signatures.

Caution Due to the proprietary nature of this complex language, we do not recommend that you edit the Service Generic engine signature parameters other than severity and event action.

The Service Generic engine allows programmatic signatures to be issued in a config-file-only signature update. It has a simple machine and assembly language that is defined in the configuration file. It runs the machine code (distilled from the assembly language) through its virtual machine, which processes the instructions and pulls the important pieces of information out of the packet and runs them through the comparisons and operations specified in the machine code. It is intended as a rapid signature response engine to supplement the String and State engines.

New functionality adds the Regex parameter to the Service Generic engine and enhanced instructions. The Service Generic engine can analyze traffic based on the mini-programs that are written to parse the packets. These mini-programs are composed of commands, which dissect the packet and look for certain conditions.

Table B-21 lists the parameters specific to the Service Generic engine.

Table B-21 Service Generic Engine Parameters

Parameter

Description

Value

Specify Dst Port

(Optional) Enables the destination port:

•Dst Port—Destination port of interest for this signature

0 to 65535

Specify IP Protocol

(Optional) Enables IP protocol:

•IP Protocol—The IP protocol this inspector should examine

0 to 255

Specify Payload Source

(Optional) Enables payload source inspection:

•Payload Source—Payload source inspection for the following types:

–Inspects ICMP data

–Inspects Layer 2 headers

–Inspects Layer 3 headers

–Inspects Layer 4 headers

–Inspects TCP data

–Inspects UDP data

ICMP Data12 Header13 Header14 HeaderTCP DataUDP Data

Specify Src Port

(Optional) Enables the source port:

•Src Port—Source port of interest for this signature

0 to 65535

Specify Regex String

The regular expression to look for when the policy type is regex:

•A regular expression to search for in a single TCP packet

•(Optional) Enables min match length for use. The minimum length of the Regex match required to constitute a match.

Regex StringSpecify Min Match Length

Swap Attacker Victim

Swaps the attacker and victim addresses and ports (source and destination) in the alert message and for any actions taken. The default is No.

Service H225 Engine

The Service H225 engine analyzes H225.0 protocol, which consists of many subprotocols and is part of the H.323 suite. H.323 is a collection of protocols and other standards that together enable conferencing over packet-based networks.

H.225.0 call signaling and status messages are part of the H.323 call setup. Various H.323 entities in a network, such as the gatekeeper and endpoint terminals, run implementations of the H.225.0 protocol stack. The Service H225 engine analyzes H225.0 protocol for attacks on multiple H.323 gatekeepers, VoIP gateways, and endpoint terminals. It provides deep packet inspection for call signaling messages that are exchanged over TCP PDUs. The Service H225 engine analyzes the H.225.0 protocol for invalid H.255.0 messages, and misuse and overflow attacks on various protocol fields in these messages.

H.225.0 call signaling messages are based on Q.931 protocol. The calling endpoint sends a Q.931 setup message to the endpoint that it wants to call, the address of which it procures from the admissions procedure or some lookup means. The called endpoint either accepts the connection by transmitting a Q.931 connect message or rejects the connection. When the H.225.0 connection is established, either the caller or the called endpoint provides an H.245 address, which is used to establish the control protocol (H.245) channel.

Especially important is the SETUP call signaling message because this is the first message exchanged between H.323 entities as part of the call setup. The SETUP message uses many of the commonly found fields in the call signaling messages, and implementations that are exposed to probable attacks will mostly also fail the security checks for the SETUP messages. Therefore, it is highly important to check the H.225.0 SETUP message for validity and enforce checks on the perimeter of the network.

The Service H225 engine has built-in signatures for TPKT validation, Q.931 protocol validation, and ASN.1PER validations for the H225 SETUP message. ASN.1 is a notation for describing data structures. PER uses a different style of encoding. It specializes the encoding based on the data type to generate much more compact representations.

You can tune the Q.931 and TPKT length signatures and you can add and apply granular signatures on specific H.225 protocol fields and apply multiple pattern search signatures of a single field in Q.931 or H.225 protocol.

•Configuration signatures for fields like ULR-ID, E-mail-ID, h323-id, and so forth for both regular expression and length.

There is a fixed number of TPKT and ASN.1 signatures. You cannot create custom signatures for these types. For TPKT signatures, you should only change the value-range for length signatures. You should not change any parameters for ASN.1. For Q.931 signatures, you can add new regular expression signatures for text fields. For SETUP signatures, you can add signatures for length and regular expression checks on various SETUP message fields.

Service HTTP Engine

The Service HTTP engine is a service-specific string-based pattern-matching inspection engine. The HTTP protocol is one of the most commonly used in networks of today. In addition, it requires the most amount of preprocessing time and has the most number of signatures requiring inspection making it critical to the overall performance of the system.

The Service HTTP engine uses a Regex library that can combine multiple patterns into a single pattern-matching table allowing a single search through the data. This engine searches traffic directed to web services only to web services, or HTTP requests. You cannot inspect return traffic with this engine. You can specify separate web ports of interest in each signature in this engine.

HTTP deobfuscation is the process of decoding an HTTP message by normalizing encoded characters to ASCII equivalent characters. It is also known as ASCII normalization.

Before an HTTP packet can be inspected, the data must be deobfuscated or normalized to the same representation that the target system sees when it processes the data. It is ideal to have a customized decoding technique for each host target type, which involves knowing what operating system and web server version is running on the target. The Service HTTP engine has default deobfuscation behavior for the Microsoft IIS web server.

(Optional) Regular expression to search in HTTP URI field. The URI field is defined to be after the HTTP method (GET, for example) and before the first CRLF. The regular expression is protected, which means you cannot change the value.

[/\\][a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z][a-zA-Z][.]jpeg

Service Ports

A comma-separated list of ports or port ranges where the target service resides.

Service IDENT Engine

The Service IDENT engine inspects TCP port 113 traffic. It has basic decode and provides parameters to specify length overflows. For example, when a user or program at computer A makes an ident request of computer B, it may only ask for the identity of users of connections between A and B. The ident server on B listens for connections on TCP port 113. The client at A establishes a connection, then specifies which connection it wants identification for by sending the numbers of the ports on A and B that the connection is using. The server at B determines what user is using that connection, and replies to A with a string that names that user. The Service IDENT engine inspects the TCP port 113 for ident abuse.

1The second number in the range must be greater than or equal to the first number.

Service MSRPC Engine

The Service MSRPC engine processes MSRPC packets. MSRPC allows for cooperative processing between multiple computers and their application software in a networked environment. It is a transaction-based protocol, implying that there is a sequence of communications that establish the channel and pass processing requests and replies.

MSRPC is an ISO Layer 5-6 protocol and is layered on top of other transport protocols such as UDP, TCP, and SMB. The MSRPC engine contains facilities to allow for fragmentation and reassembly of the MSRPC PDUs. This communication channel is the source of recent Windows NT, Windows 2000, and Window XP security vulnerabilities. The Service MSRPC engine only decodes the DCE and RPC protocol for the most common transaction types.

Service MSSQL Engine

The Service MSSQL engine inspects the protocol used by the Microsoft SQL server. There is one MSSQL signature. It fires an alert when it detects an attempt to log in to an MSSQL server with the default sa account. You can add custom signatures based on MSSQL protocol values, such as login username and whether a password was used.

Service NTP Engine

The Service NTP engine inspects NTP protocol. There is one NTP signature, the NTP readvar overflow signature, which fires an alert if a readvar command is seen with NTP data that is too large for the NTP service to capture. You can tune this signature and create custom signatures based on NTP protocol values, such as mode and size of control packets.

•Control Opcode—Opcode number of an NTP control packet according to RFC1305, Appendix B.

•Max Control Data Size—Maximum allowed amount of data sent in a control packet.

•Operation Mode—Mode of operation of the NTP packet per RFC 1305.

0 to 65535

IS Invalid Data Packet

Looks for invalid NTP data packets. Checks the structure of the NTP data packet to make sure it is the correct size.

Yes | No

Is Non NTP Traffic

Checks for nonNTP packets on an NTP port.

Yes | No

Service P2P

P2P networks use nodes that can simultaneously function as both client and server for the purpose of file sharing. P2P networks often contain copyrighted material and their use on a corporate network can violate company policy. The Service P2P engine monitors such networks and provides optimized TCP and UDP P2P protocol identification. The Service P2P engine has the following characteristics:

•Listens on all TCP and UDP ports

•Increased performance through the use of hard-coded signatures rather than regular expressions

•Ignores traffic once P2P protocol is identified or after seeing 10 packets without a P2P protocol being identified

Because the P2P signatures are hard coded, the only parameters that you can edit are the Master engine parameters.

Service RPC Engine

The Service RPC engine specializes in RPC protocol and has full decode as an anti-evasive strategy. It can handle fragmented messages (one message in several packets) and batch messages (several messages in a single packet).

The RPC portmapper operates on port 111. Regular RPC messages can be on any port greater than 550. RPC sweeps are like TCP port sweeps, except that they only count unique ports when a valid RPC message is sent. RPC also runs on UDP.

Service SMB Advanced Engine

Caution The SMB engine has been replaced by the SMB Advanced engine. Even though the SMB engine is still visible in IDM, IME, and the CLI, its signatures have been obsoleted; that is, the new signatures have the obsoletes parameter set with the IDs of their corresponding old signatures. Use the new SMB Advanced engine to rewrite any custom signature that were in the SMB engine.

The Service SMB Advanced engine processes Microsoft SMB and Microsoft RPC over SMB packets. The Service SMB Advanced engine uses the same decoding method for connection-oriented MSRPC as the MSRPC engine with the requirement that the MSRPC packet must be over the SMB protocol. The Service SMB Advanced engine supports MSRPC over SMB on TCP ports 139 and 445. It uses a copy of the connection-oriented DCS/RPC code from the MSRPC engine.

Table B-29 lists the parameters specific to the Service SMB Advanced engine.

Table B-29 Service SMB Advanced Engine Parameters

Parameter

Description

Value

Service Ports

A comma-separated list of ports or port ranges where the target service resides.

Service SNMP Engine

The Service SNMP engine inspects all SNMP packets destined for port 161. You can tune SNMP signatures and create custom SNMP signatures based on specific community names and object identifiers.

Instead of using string comparison or regular expression operations to match the community name and object identifier, all comparisons are made using the integers to speed up the protocol decode and reduce storage requirements.

•Bruce Force Count—The number of unique SNMP community names that constitute a brute force attempt.

0 to 65535

Invalid Packet Inspection

Inspects for SNMP protocol violations.

—

Non SNMP Traffic Inspection

Inspects for non-SNMP traffic destined for UDP port 161.

—

SNMP Inspection

Inspects SNMP traffic:

•Specify Community Name [yes | no]:

–Community Name—Searches for the SNMP community name, that is, the SNMP password.

•Specify Object ID [yes | no]:

–Object ID—Searches for the SNMP object identifier.

community-name

object-id

Service SSH Engine

The Service SSH engine specializes in port 22 SSH traffic. Because all but the setup of an SSH session is encrypted, the engine only looks at the fields in the setup. There are two default signatures for SSH. You can tune these signatures, but you cannot create custom signatures.

•Packet Depth—Number of packets to watch before determining the session key was missed.

0 to 65535

1The second number in the range must be greater than or equal to the first number.

Service TNS Engine

The Service TNS engine inspects TNS protocol. TNS provides database applications with a single common interface to all industry-standard network protocols. With TNS, applications can connect to other database applications across networks with different protocols. The default TNS listener port is TCP 1521. TNS also supports REDIRECT frames that redirect the client to another host and/or another TCP port. To support REDIRECT packets, the TNS engine listens on all TCP ports and has a quick TNS frame header validation routine to ignore non-TNS streams.

State Engine

The State engine provides state-based regular expression-based pattern inspection of TCP streams. A state engine is a device that stores the state of something and at a given time can operate on input to transition from one state to another and/or cause an action or output to take place. State machines are used to describe a specific event that causes an output or alert. There are three state machines in the State engine: SMTP, Cisco Login, and LPR Format String.

String Engines

Understanding String Engines

The String engine is a generic-based pattern-matching inspection engine for ICMP, TCP, and UDP protocols. The String engine uses a regular expression engine that can combine multiple patterns into a single pattern-matching table allowing for a single search through the data. There are three String engines: String ICMP, String TCP, and String UDP.

Sweep Engines

Sweep Engine

The Sweep engine analyzes traffic between two hosts or from one host to many hosts. You can tune the existing signatures or create custom signatures. The Sweep engine has protocol-specific parameters for ICMP, UDP, and TCP.

The alert conditions of the Sweep engine ultimately depend on the count of the unique parameter. The unique parameter is the threshold number of distinct hosts or ports depending on the type of sweep. The unique parameter triggers the alert when more than the unique number of ports or hosts is seen on the address set within the time period. The processing of unique port and host tracking is called counting.

Caution Event action filters based on source and destination IP addresses do not function for the Sweep engine, because they do not filter as regular signatures. To filter source and destination IP addresses in sweep alerts, use the source and destination IP address filter parameters in the Sweep engine signatures.

A unique parameter must be specified for all signatures in the Sweep engine. A limit of 2 through 40 (inclusive) is enforced on the sweeps. 2 is the absolute minimum for a sweep, otherwise, it is not a sweep (of one host or port). 40 is a practical maximum that must be enforced so that the sweep does not consume excess memory. More realistic values for unique range between 5 and 15.

TCP sweeps must have a TCP flag and mask specified to determine which sweep inspector slot in which to count the distinct connections. The ICMP sweeps must have an ICMP type specified to discriminate among the various types of ICMP packets.

Data Node

When an activity related to Sweep engine signatures is seen, the IPS uses a data node to determine when it should stop monitoring for a particular host. The data node contains various persistent counters and variables needed for cross-packet reassembly of streams and for tracking the inspection state on a per-stream/per-source/per-destination basis The data node containing the sweep determines when the sweep should expire. The data node stops a sweep when the data node has not seen any traffic for x number of seconds (depending on the protocol).

There are several adaptive timeouts for the data nodes. The data node expires after 30 seconds of idle time on the address set after all of the contained objects have been removed. Each contained object has various timeouts, for example, TCP Stream has a one-hour timeout for established connections. Most other objects have a much shorter expiration time, such as 5 or 60 seconds.

Does not fire when a sweep has fired in the reverse direction on this address set.

Yes | No

Swap Attacker Victim

Swaps the attacker and victim addresses and ports (source and destination) in the alert message and for any actions taken. The default is No.

Yes | No

TCP Flags

TCP flags to match when masked by mask:

•URG bit

•ACK bit

•PSH bit

•RST bit

•SYN bit

•FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Unique

Threshold number of unique port connections between the two hosts.

0 to 65535

Sweep Other TCP Engine

The Sweep Other TCP engine analyzes traffic between two hosts looking for abnormal packets typically used to fingerprint a victim. You can tune the existing signatures or create custom signatures.

TCP sweeps must have a TCP flag and mask specified. You can specify multiple entries in the set of TCP flags. And you can specify an optional port range to filter out certain packets.

Table B-38 lists the parameters specific to the Sweep Other TCP engine.

Table B-38 Sweep Other TCP Engine Parameters

Parameter

Description

Value

Specify Port Range

(Optional) Enables using a port range for inspection:

•Port Range—UDP port range used in inspection.

0 to 65535a-b[,c-d]

Set TCP Flags

Lets you set TCP flags to match.

•TCP Flags—TCP flags used in this inspection:

–URG bit

–ACK bit

–PSH bit

–RST bit

–SYN bit

–FIN bit

•URG

•ACK

•PSH

•RST

•SYN

•FIN

Traffic Anomaly Engine

The Traffic Anomaly engine contains nine anomaly detection signatures covering the three protocols (TCP, UDP, and other). Each signature has two subsignatures, one for the scanner and the other for the worm-infected host (or a scanner under worm attack). When anomaly detection discovers an anomaly, it triggers an alert for these signatures. All anomaly detection signatures are enabled by default and the alert severity for each one is set to high.

When a scanner is detected but no histogram anomaly occurred, the scanner signature fires for that attacker (scanner) IP address. If the histogram signature is triggered, the attacker addresses that are doing the scanning each trigger the worm signature (instead of the scanner signature). The alert details state which threshold is being used for the worm detection now that the histogram has been triggered.

From that point on, all scanners are detected as worm-infected hosts.

The following anomaly detection event actions are possible:

•Produce alert—Writes the event to the Event Store.

•Deny attacker inline—(Inline only) Does not transmit this packet and future packets originating from the attacker address for a specified period of time.

Identified a worm attack over a TCP protocol in the internal zone; the TCP histogram threshold was crossed and a scanner over a TCP protocol was identified.

13001

0

Internal UDP Scanner

Identified a single scanner over a UDP protocol in the internal zone.

13001

1

Internal UDP Scanner

Identified a worm attack over a UDP protocol in the internal zone; the UDP histogram threshold was crossed and a scanner over a UDP protocol was identified.

13002

0

Internal Other Scanner

Identified a single scanner over an Other protocol in the internal zone.

13002

1

Internal Other Scanner

Identified a worm attack over an Other protocol in the internal zone; the Other histogram threshold was crossed and a scanner over an Other protocol was identified.

13003

0

External TCP Scanner

Identified a single scanner over a TCP protocol in the external zone.

13003

1

External TCP Scanner

Identified a worm attack over a TCP protocol in the external zone; the TCP histogram threshold was crossed and a scanner over a TCP protocol was identified.

13004

0

External UDP Scanner

Identified a single scanner over a UDP protocol in the external zone.

13004

1

External UDP Scanner

Identified a worm attack over a UDP protocol in the external zone; the UDP histogram threshold was crossed and a scanner over a UDP protocol was identified.

13005

0

External Other Scanner

Identified a single scanner over an Other protocol in the external zone.

13005

1

External Other Scanner

Identified a worm attack over an Other protocol in the external zone; the Other histogram threshold was crossed and a scanner over an Other protocol was identified.

13006

0

Illegal TCP Scanner

Identified a single scanner over a TCP protocol in the illegal zone.

13006

1

Illegal TCP Scanner

Identified a worm attack over a TCP protocol in the illegal zone; the TCP histogram threshold was crossed and a scanner over a TCP protocol was identified.

13007

0

Illegal UDP Scanner

Identified a single scanner over a UDP protocol in the illegal zone.

13007

1

Illegal UDP Scanner

Identified a worm attack over a UDP protocol in the illegal zone; the UDP histogram threshold was crossed and a scanner over a UDP protocol was identified.

13008

0

Illegal Other Scanner

Identified a single scanner over an Other protocol in the illegal zone.

13008

1

Illegal Other Scanner

Identified a worm attack over an Other protocol in the illegal zone; the Other histogram threshold was crossed and a scanner over an Other protocol was identified.

Traffic ICMP Engine

The Traffic ICMP engine analyzes nonstandard protocols, such as TFN2K, LOKI, and DDoS. There are only two signatures (based on the LOKI protocol) with user-configurable parameters.

TFN2K is the newer version of the TFN. It is a DDoS agent that is used to control coordinated attacks by infected computers (zombies) to target a single computer (or domain) with bogus traffic floods from hundreds or thousands of unknown attacking hosts. TFN2K sends randomized packet header information, but it has two discriminators that can be used to define signatures. One is whether the L3 checksum is incorrect and the other is whether the character 64 `A' is found at the end of the payload. TFN2K can run on any port and can communicate with ICMP, TCP, UDP, or a combination of these protocols.

LOKI is a type of back door Trojan. When the computer is infected, the malicious code creates an ICMP Tunnel that can be used to send small payload in ICMP replies (which may go straight through a firewall if it is not configured to block ICMP.) The LOKI signatures look for an imbalance of ICMP echo requests to replies and simple ICMP code and payload discriminators.

The DDoS category (excluding TFN2K) targets ICMP-based DDoS agents. The main tools used here are TFN and Stacheldraht. They are similar in operation to TFN2K, but rely on ICMP only and have fixed commands: integers and strings.

Inbalance of replies to requests. The alert fires when there are this many more replies than requests.

0 to 65535

Want Request

Requires an ECHO REQUEST be seen before firing the alert.

Yes | No

Trojan Engines

The Trojan engines analyze nonstandard protocols, such as BO2K and TFN2K. There are three Trojan engines: Trojan BO2K, TrojanTFN2K, and Trojan UDP.

BO was the original Windows back door Trojan that ran over UDP only. It was soon superseded by BO2K. BO2K supported UDP and TCP both with basic XOR encryption. They have plain BO headers that have certain cross-packet characteristics.

BO2K also has a stealthy TCP module that was designed to encrypt the BO header and make the cross-packet patterns nearly unrecognizable. The UDP modes of BO and BO2K are handled by the Trojan UDP engine. The TCP modes are handled by the Trojan BO2K engine.

Note There are no specific parameters to the Trojan engines, except for Swap Attacker Victim in the Trojan UDP engine.